In a conventional electric power conversion apparatus, mechanical relays are used to shut off electric connection between motor phase terminals of a three-phase electric motor and output terminals of an inverter circuit. If such a mechanical relay is used, reliability of its operation is lowered by foreign matters and the like, which are often jammed. The following patent document therefore proposes to use a power MOS-type field effect transistor in place of such a mechanical relay.
[Patent document] JP 2006-21645A
According to this proposal, the transistors are connected between a three-phase electric motor and an inverter circuit so that electrical connection and disconnection therebetween is controlled by a voltage applied to the gate of the transistor.
If a current required by the electric motor is increased, the amount of heat generated by the transistor tends to excessively increase in a period, in which the transistor is turned on to electrically connect the inverter circuit and the electric motor. If the heat generated by the transistor excessively increases, the reliability of the transistor will be lowered.